That doesn't mean anything, it's stolen properly. As far as I know, ignorance of the source of an item doesn't matter. You buy stolen merchandise, tough luck. Stolen is stolen. Ant would probably know for sure, though, I could be totally wrong. And besides, Gamestop should have gotten the guy's address or something... so unless that was false and he gave a false name, I'd think the cops should be able to track him down.
And really, who the fuck would steal games just to trade them in? Games are worth practically nothing that way, even new.